1. Field of the Invention
The invention is directed to tin catalysts to promote the low temperature cure of blocked isocyanates and blocked isothiocyanates.
2. Description of Related Art
Organotin compounds, particularly diorganotins, are commonly used for the curing reaction of blocked isocyanates with hydroxyl-containing compounds. These systems find applications in coatings, where the hydroxyl-containing compound is polymeric and the blocked isocyanate is multifunctional. Frequently, the blocking agent is an aliphatic alcohol, which imparts long pot life in one pot systems. Reaction of the pendant hydroxyl groups with the multifunctional blocked isocyanate occurs at elevated temperatures in a cross-linking reaction which, increases the molecular weight and results in a cured coating which has excellent solvent resistance.
Due to environmental considerations, the coatings industry has been turning to systems wherein the reactants are dispersed in an aqueous system. These systems require the formation of stable dispersions and hydrolytic stability for all reactants. In particular, primer coatings may be deposited from aqueous dispersion onto metal surfaces by cathodic deposition, as described by Bosso et al., U.S. Pat. No. 4,101,486.
Catalysts are usually needed in order to promote the curing reaction when the blocking agent is an aliphatic alcohol. Conventionally, these catalysts are stannous salts or mono- or diorganotin compounds which catalyze the curing or cross-linking reaction at temperatures in the range 330.degree.-365.degree. F. It is often desirable to obtain curing reactions at lower temperatures in order to conserve energy, reduce deformation of plastic parts attached to the metal object, and reduce color formation.
Thiele et al., Plaste und Kautschuk, 36 January 1989 (1) pp. 1-3, disclose the reaction of phenylisocyanate and butanol in the presence of bis tributyltin oxide as a model reaction for urethane polymers wherein the addition of one mol percent water retarded the rate of reaction and caused a deviation in the linearity in Eyring diagrams. The retarding effect of the water may be reduced by increasing the temperature. The reference suggests that triorganotin catalysts are not suitable in aqueous systems where lower temperature cures are required.
Jerabek U.S. Pat. No. 4,031,050, Jerabek et al., U.S. Pat. No. 4,017,438 and Bosso et al. describe aqueous coating compositions based on blocked organic polyisocyanates, an amine adduct of an epoxy group-containing resin and a diorganotin catalyst. These compositions are cationic and may be electrodeposited on a cathode and are widely employed as primers for automotive substrates. In this process, a conductive article such as an auto body or an auto part is immersed in a bath of the aqueous coating and acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous coating until a desired coating thickness is deposited on the article.
The amine adduct of the epoxy resin is cationic and is readily coated on cathodic metal substrates such as auto bodies or auto parts. The coating operation may be conducted as a continuous process which requires the bath to be monitored and replenished periodically with the coating composition and/or components of the composition which are depleted as successive coating operations are carried out.
The diorganotin catalyst employed is a solid that is dispersed in the coating composition and in some instances will separate from the coating and deposit, with other coating residues, on the bottom of the tank which contains the coating bath. The amount of catalyst in the bath, therefore can be depleted requiring that it be replenished so that the cure of the coating is effected in a timely manner. Replenishing the solid catalyst can be difficult or a disadvantage since it has to be properly dispersed in a suitable medium before being introduced into the bath.
Although these cationic amine adducts of the epoxy resin can be formulated with pigments and/or fillers, attempts are being made to provide coating systems that do not have any solid materials in them as a cost savings measure and also to eliminate various problems in the coating tank with solid materials settling to the bottom of the tank which include solid organotin compounds. These materials that settle have to be separated by an ultrafiltration process and where catalyst is removed in this process, it has to be replaced. An essentially solids-free coating system would therefore be desirable to avoid or minimize the settling problem. Additionally, the expense of preparing such a coating could be reduced by eliminating any grinding step that would be required to disperse catalysts and/or pigments, fillers and the like in the coating composition.
Coatings without pigments or fillers can be used as first coats in several applications where subsequent coats would provide the pigment materials that are in some instances necessary to protect the coating from ultraviolet radiation or other environmental hazards that could cause the coating to deteriorate at an unacceptable rate.
If coatings of this type can be applied electrolytically at a faster rate as well as cured at a faster rate, an increase in production rates would be obtained which represent a cost savings to the manufacturer.
Although the cationic amine-epoxy resins can be applied to metallic substrates electrolytically, these types of coatings are self-limiting by which it is meant that after a certain thickness, the coating build up slows and eventually stops since the coating material is insulating. Higher build coatings are an advantage since equivalent coating thicknesses can be applied more quickly or the full thickness of the coating can be obtained to provide improved physical properties such as impact resistance, corrosion resistance and the like.
Additionally, one problem encountered with some prior art coatings of this type is the inability to obtain a sufficient coating thickness at the edge of the object being coated. Edges, with this reduced coating tend to wear or corrode faster and can be regions on the metal article where a loss of structural integrity will occur first.
It is also desirable to eliminate pigments and/or fillers in coating compositions of this type since they are a source of pinholing in the coating which compromises the integrity of the coating layer and consequently exposes the metal substrate to wear and corrosion.
It would therefore be an advantage to obtain a catalyst that would promote the cure of these type of coatings at substantially the same rate as the catalyst presently used and which would be easily incorporated into the coating composition and would not tend to separate during use. Catalyst that are liquids at coating conditions and which are either soluble or readily dispersible, i.e., emulsified in the coating composition would be especially preferred in this regard.
Chung et al. U.S. Pat. No. 5,116,914 notes that dibutyltin oxide, which is used as a catalyst in these aqueous coatings is difficult to disperse whereas dibutyltin dilaurate can be hydrolyzed which causes cratering problems in the deposited film. The patentees describe the use of a dibutyltin diacetyl acetonate catalyst to avoid these problems.
Treadwell et al. U.S. Pat. No. 4,032,468 describes the use of a trimethyl or a trimethylmethoxytin oxide catalyst for the preparation of hydrolytically stable urethane foam precursors. The foam is formed by the reaction of the isocyanate component of the urethane foam with water.
Coe U.S. Pat. No. 4,286,073 describes the use of tributyltin toluenesulfonate or methanesulfonate catalysts for the manufacture of urethanes whereas Groves, U.S. Pat. No. 4,087,412 teaches a mixture of trialkyltin oxide and a reaction product of a carboxylic acid and a dialkyl tin oxide catalyst for the formation of polyurethane polymers. Zemlin, U.S. Pat. No. 3,523,103 describes the use of a tri-organoditin catalyst for the formation of polyurethanes.
Accordingly, catalysts that would not detract from the stability of the electrolytic bath employed according to the Jerabek, Jerabek et al. and Bosso et al. patents would be advantageous. Additionally, it would be an advantage to provide a catalyst that had improved throwing power in such baths, i.e., an increase in the amount of coating deposited in remote areas. Catalysts that also promote the deposition of coatings from these baths at a lower weight but afford equivalent protection as do heavier coatings are also desirable. When used as automotive coatings, this would result in some reduction in automobile weight leading to some measure of emission reduction and improvement in fuel economy. Other properties which are sought in these types of catalysts include improved ultrafiltration, reduced grind preparation, increased deposition rate, improved dispersability or emulsifiability, reduced cure temperatures, easier handling, improved color maintenance and a lower level of catalyst used.